A transimpedance amplifier is an amplifier that converts current to voltage. Its input ideally has zero impedance, and the input signal is a current. Its output may have low impedance. The output signal is measured as a voltage. As the output is a voltage and the input is a current, the gain, or ratio of output to input, is expressed in units of ohms.
Transimpedance amplifiers are commonly used in receivers for optical communications. The photodetector that generates the transimpedance amplifier's input current owns a capacitance. This capacitance in conjunction with the transimpedance amplifier's input impedance poses as a low pass filter, which limits the bandwidth of the signal chain. Therefore the transimpedance amplifiers input impedance should be as low as possible.
Transimpedance amplifiers are tested during production to guarantee their successful operation. According to one example known in the state of the art, a transimpedance amplifier comprises a first current input for the current of a photodiode and a second input for a test current. Both inputs may be connected in parallel with each other and to the inverting input of an operational amplifier. For testing the successful operation, a test current is applied to the test current input, in particular in form of a transient test current.
One disadvantage of the testing of operation of transimpedance amplifiers having a high gain ratio is that applying the test current can result in an undesired oscillation of the amplifier.
To be able to apply a test current to a transimpedance amplifier, a test current source is needed. The test current source itself has an inductance that can interact with the transimpedance amplifier and therefore causes the undesired oscillations. To damp these oscillations, it is known in the state of the art to provide a connection in series of the test current source and a resistor. The test input connected in parallel with the first input adds capacitance to the signal chain wherein said capacitance can degrade the performance of the transimpedance amplifier.
It is therefore one objective of the present disclosure to overcome the problems of oscillations of transimpedance amplifiers during testing and of a reduction of their performance during normal operation. Another objective of the present disclosure is to provide an apparatus and a method for improving the testing of transimpedance amplifiers.